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Daniel Arnold, Esq. April 6, 2011 <br />Denver Water Page 3 of 12 <br />3) Historic mine spoils are said to have been placed in the south side of the creek <br />before the waste rock piles were constructed, and these spoils are in contact with <br />groundwater and likely to be the source of the uranium in groundwater. These <br />historic spoils could be a contributing source, but do not preclude the larger waste <br />rock piles as sources and additional investigation is needed. Regardless, either <br />historic or the more recent mine wastes in the waste rock piles, are source(s) of <br />uranium in groundwater that need to be addressed. <br />Alluvial Fill <br />Recent information collected by Cotter as part of the TR -14 source -term characterization <br />project shows that waste rock placed as construction material within the valley has <br />elevated uranium that is in contact with groundwater. This is new information that was <br />not in the EPP, and if correct indicates that this "alluvial fill" material is probably <br />comprised of more waste rock than inert fill and either removal or isolation is needed. <br />Data from the TR -14 project should be made available and a plan for removal actions <br />should be prepared. <br />Uranium concentrations in the alluvial groundwater are stated as being highly variable <br />prior to mine pool flooding. The validity of this statement cannot be determined. <br />Historic groundwater quality data for several of the wells dating back to as early as 1989 <br />is not contained in the EPP, nor has it been provided in the comments to substantiate this <br />statement. <br />An argument is presented to explain why alluvial groundwater concentrations may have <br />been lower prior to termination of the mine dewatering, claiming that treated water was <br />discharged to the creek and this clean water may have diluted groundwater <br />concentrations. The relatively low discharge rate from water treatment would have <br />provided minimal dilution water as compared to what the creek was providing. The <br />argument states that groundwater concentrations were lower prior to terminating mine <br />dewatering, which contradicts what is stated in the EPP. <br />Mine Pool <br />A comment states that the EPP Review does not explain why there is an increase in <br />uranium concentrations in MW -6 and MW -7 from 2001 through 2003 (see Figure 4 of <br />the EPP Review), and that the increase is not due to the mine pool because the mine pool <br />did not reach the elevation of the wells until 2006. This observation may be correct. <br />However, the increase in uranium during the 2002 through 2003 period can be explained <br />by leachate from the waste rock pile area. Examination of the uranium concentrations in <br />MW -0, which is at the toe of the south waste rock pile (see Figure 2 of the EPP Review), <br />shows a similar concentration pattern as MW -6 and MW -7. The uranium concentration <br />in MW -0 was at 1 mg/L in early 2002, which is similar to the concentrations in the other <br />two wells at that time, indicating that the upgradient waste rock piles could have been the <br />